The present invention relates to a mechanism for rotationally driving a member intermittently, for example for driving a day of the month indicator for watches having an analog display and, more especially, for electronic watches.
It is well known that in analog watches, the indication of the day of the month is effected conventionally by a disk or ring which bears the numbers 1 to 31, these numbers passing successively in front of a window formed in the dial of the watch. This indicator is rotationally driven by the gear train in such a manner that it advances one step every twenty-four hours. The locking in of each of the display positions of the day of the month indicator is usually effected by a resilient detent which engages successively between the different teeth of a set of teeth formed on the disk or the ring, as the indicator advances. During the daily actuation of the indicator, the motive force which needs to be applied to it is thus distinctly greater than the motive force that is required for only driving the hands of the watch, since it is necessary to furnish in addition the energy for withdrawing the resilient detent.
This excess of necessary energy, which is only very temporary since it is only produced once per 24 hours, is particularly prejudicial in the case of electronic watches. The motor has to be powerful enough to furnish the torque necessary for driving the indicator. In order to avoid having to supply constantly to the motor the amount of energy necessary to furnish this torque, it is possible to provide the motor with a control circuit which adapts the energy supplied to the motor to the load and hence in particular to the torque which needs to be applied to the day of the month indicator. However, such a circuit complicates the control circuit of the watch.
Accordingly, attempts have been made to reduce the torque which has to be applied to the day of the month indicator in order to make it advance one step.
Swiss Pat. No. 538.136 describes such a mechanism which enables the torque needed to drive the day of the month indicator to be reduced. In this mechanism, the detent for resiliently locking the day of the month indicator in its different positions is eliminated, whereby the torque that needs to be applied is reduced. But this mechanism has other disadvantages. Such a device comprises a driving disk, an intermediate plate provided with pins which cooperate with the disk and a day of the month indicating disk which is provided on its inner periphery with deep recesses. The intermediate plate is provided with two pins which can enter the recesses in the day of the month disk and can also cooperate with a projection and two recesses formed respectively on and in the periphery of the driving disk. The disk thus drives the day of the month disk one step during each of its rotations by way of the plate provided with its two pins. The locking of the disk is ensured by the presence of two supplementary studs mounted on the disk. Outside the driving phases, one of the studs and one of the pins engage the edge surface of the disk thus ensuring the immobilization of the disk while permitting a free rotation of the driving disk. This solution has serious disadvantages because of the cooperation between the pins and the slots formed in the day of the month disk and between these same pins and the projection formed on the driving disk. It is necessary to provide a substantial amount of play in order for the device to be able to function and the immobilization of the disk is therefore mediocre. In addition, these amounts of play give rise to supplementary shocks which consume energy and cause wear. Besides, according to the same document, in order to ensure an effective locking of the day of the month ring, there is provided a permanent magnet which effectively retains the ring and the plate that carries the studs in position.